1. Field of the Invention
The present invention relates to a linear-motion device such as linear guide device and ball screw to be incorporated in industrial machines, etc. and more particularly to a technique for reducing noise in and prolonging the life of such a linear-motion device.
The present invention relates to a ball screw which has balls to be circulated endlessly in a return tube process and more particularly to a low noise ball screw which is preferably used in machine tools, industrial robots, etc.
In particular, the present invention relates to a ball screw which is preferably used in high speed precision positioning in machine tool bonder, etc.
2. Discussion of Related Arts
As shown in FIG. 1, a linear guide device 10 comprises a guide rail 1 having a rolling groove 3 provided on the outer surface thereof and a slider 2 mounted across the guide rail 1. The side of the slider 2 facing the rolling groove 3 of the guide rail 1 is partly opened to form a ball circulating path 4 in the form of racing track with the rolling groove 3 of the guide rail 1. Into the ball circulating path 4 are received rollably a large number of balls B.
Alternatively, as shown in FIG. 2, a ball screw device 20 comprises a ball nut 12 provided surrounding a screw shaft 11, and a plurality of balls B rollably disposed in the space defined by a thread groove 12a formed spirally on the inner surface of the ball nut 12 and a thread groove 11a formed spirally on the outer surface of the screw shaft 11 opposed to the thread groove 12a. The ball nut 12 has a ball tube 13 having an external shape of U mounted with its both ends extending to the thread groove 11a of the screw shaft 11. The balls B make a repeated circulation. In some detail, the balls B run around the screw shaft 11 plural times inside the ball nut 12. The balls B are then caught by one end of the ball tube 13 from which they then pass through the ball circulating path 18. The balls B are then returned to the thread groove 11a of the screw shaft 11 from the other end of the ball tube 13.
The linear guide device 10 and the ball screw 20 each normally have a separator provided interposed between the balls B to eliminate sound developed by the collision of balls during driving. FIG. 3 is an enlarged view of the interior of the ball circulating path 4 of the linear guide device 10 illustrating a series of balls B with a separator 100 provided interposed therebetween. The separator 100 has a concave surface 101 having an arc section corresponding to the outer surface of the ball B. The ball B is rollably retained on the concave surface 101 during circulation through the ball circulating path 4.
The separator 100 is a formed product of resin composition which is unreinforced or comprises a proper reinforcing material incorporated therein. As such a resin there is normally used a polyamide resin such as nylon 66.
However, a polyamide resin is a hard plastic having a high rigidity. Thus, the separator 100 made of such a polyamide resin undergoes a small deformation due to collision with the ball B during the operation of the linear-motion device. Therefore, the separator 100 exhibits an insufficient sound absorbing quality and cannot deform sufficiently when passing through the curved ball circulating path 4 together with the balls B, causing the deterioration of operating characteristics. Actually, frictional force momentarily rises during operation, making it impossible to obtain good operating characteristics invariably.
The recent trend is for more polyester-based elastomers comprising as a hard segment a polybutylene terephthalate (PBT) to be used Taking into account ease of assembly or sound absorbing quality.
The polyester-based elastomer comprising PBT as a hard segment exhibits a lower water absorption than polyamide and hence an excellent dimensional stability and has an excellent assembly and sound absorbing quality but is disadvantageous in that it has a deteriorated resistance to grease to be filled into the device for lubrication. Therefore, the separator 100 swells outward with time. At the same time, the position on the concave surface 101 in contact with the ball B deviates toward the center of the concave surface 101, causing the gap between the ball B and the separator 100 to increase gradually and hence raising the noise level. In some cases, as shown in FIG. 4, the separator 100 may fall outside the ball. The separator 100 which has thus fallen can prevent the ball B from circulating, making it impossible to drive the device.
On the other hand, in general, a ball screw is disadvantageous in that balls can difficultly move smoothly through a path defined by the thread groove of the screw shaft and the thread groove of the nut (hereinafter referred to as “ball rolling path”) and a return tube.
In particular, when the balls are caught by the return tube from the ball rolling path, there is a tendency that the balls can difficultly move smoothly at the border of the opening at the end of the return tube (i.e., ball catching point) with the ball rolling path (hereinafter referred to as “ball catch starting point”).
When the movement of the balls cannot be effected smoothly at the ball catch starting point, slight vibration occurs as the balls pass through the ball catch starting point (hereinafter referred to as “vibration due to passage of balls”), preventing the balls from circulating fairly and hence deteriorating the precision in feeding. This also causes the occurrence of noise.
In order to solve these problems, various ball screws designed to allow the balls to pass through the ball catch starting point smoothly have been proposed.
For example, Japanese Utility Model Publication No. 05-27729 discloses a ball screw having the both ends of a return tube disposed in the direction of lead angle.
A ball screw having the both ends of a return tube disposed in the direction tangential to the ball track has heretofore been known. A ball screw having the both ends of a return tube disposed in both the direction tangential to the ball track and the direction of lead angle is disclosed in Japanese Utility Model Laid-Open No. 06-69502.
Further, Japanese Patent No. 2832943 discloses a ball screw comprising a thread groove on the nut the lead angle of which gradually changes so that the pilot pressure on the balls gradually decreases toward the ball catch starting point and is released at the ball catch starting point in the ball rolling path in the vicinity of the ball catch starting point.
Moreover, Japanese Patent Publication No. 57-38829 discloses a ball screw comprising a thread groove on the nut the diameter of which gradually increases so that the constraint on the balls is gradually released toward the ball catch starting point and is completely released at the ball catch starting point in the ball rolling path in the vicinity of the ball catch starting point.
However, the aforementioned four examples contemplate smooth movement of balls only at a part of the portion where the balls move and thus cannot be a complete solution to the aforementioned problems. These approaches leave something to be desired particularly with ball screws which are used at a high feed rate or high rotary speed.
In other words, the ball screws disclosed in Japanese Utility Model Publication No. 05-27729 and Japanese Utility Model Laid-Open No. 06-69502 allow the ball's to move smoothly through the return tube but give no consideration to the movement of the balls through the ball rolling path.
Further, the ball screws disclosed in Japanese Patent No. 2832943 and Japanese Patent Publication No. 57-38829 allow the balls to move smoothly through the ball rolling path (nut side) as opposed to the aforementioned approaches but give no give no consideration to the movement of the balls through the return tube. Accordingly, when there is a difference between the direction of movement of balls through the ball rolling path and the direction of the end of the return tube, the halls collide with the inner wall of the return tube the moment the balls move from the ball rolling path to the return tube, making it difficult for the balls to move smoothly.
Moreover, since the return tube normally has a portion which is curved substantially at the right angle, it is likely that the balls can be prevented from moving smoothly at the bent portion.
In addition to this, a ball screw comprises a screw shaft and a plurality of balls disposed on a spiral track formed between the screw shaft and a nut in which the screw shaft is disposed and is adapted to transmit power between the screw shaft and the nut with the rolling and circulation of the balls. A spiral groove is provided both on the outer surface of the screw shaft and the inner surface of the nut.
The diameter of the balls depends on the diameter and lead (distance of movement of the nut per rotation of the screw shaft) of the screw shaft. The size of the balls is increased depending on the lead of the screw shaft to obtain a sufficient nominal dynamic load.
Heretofore, the ratio of the ball diameter to the radius of curvature of section of groove (hereinafter also referred to as “radius of curvature of groove”) has been predetermined taking into account the value concerning rolling bearings rather than the frictional characteristics of ball screw. In order to cope with competitive friction of balls due to dispersion of revolution speed of balls, it has been practiced to provide a spacer ball interposed between the balls.
Japanese Patent Laid-Open No. 2000-39052 proposes the same measure as proposed for ball bearing, i.e., predetermination of the radius of curvature of groove on the screw shaft side to be smaller than the radius of curvature of groove on the nut side for the purpose of rendering face pressure uniform to prolong the life of the screw shaft.
A precision ball screw (precision positioning ball screw) comprises a groove formed in Gothic arch. The gap in the axial direction is predetermined to be as small as about one hundredths of the ball diameter. The balls can make three-point contact even only at axial load during driving, making it possible to obtain a stable rigidity. However, much sliding heat is generated at the three contact areas during driving. This sliding head predominates in the heat generation of the entire ball screw and is a factor lowering the efficiency and life of ball screw.
In order to inhibit the heat generation in the three contact areas, it can be proposed to increase the gap in the axial direction. However, this proposal is not desirable from the stand point of precision and acoustic troubles such as generation of abnormal sound. Further, even when the ball diameter is raised somewhat to increase the nominal dynamic load, the life of the ball screw cannot be prolonged unless the friction is reduced because the life of ball screw is actually determined by surface starting point fracture due to frictional work. The radius of curvature of groove of related art precision ball screws merely follows the value concerning rolling bearings but doesn't take into account the heat generation due to three-point contact.
Problems often arise with life and heat generation involving precision in positioning due to frictional wear more than with face pressure discussed in the above cited Japanese Patent Laid-Open No. 2000-39052. The radius of curvature of groove on the nut and screw shaft sides has never been predetermined taking into account these factors.